1. Field of the invention
The present invention relates to a mirror control apparatus for a mirror in a single lens reflex camera.
2. Description of the Related Art
A single lens reflex (SLR) camera has a mirror disposed in a camera body and supported for reciprocal rotation between a first position in a photographing light path and a second position out of the photographing light path. The mirror is normally positioned at the first position, at which it is angled at forty-five degrees, serving to reflect light coming through the lens onto the viewing screen of a viewfinder.
The mirror is driven by a mirror control apparatus for quick rotation from the first to the second position and then from the second to the first position, so that it tends to impact onto a part of the camera body when reaching either of the positions. In order to prevent any harmful shocks to the mirror and/or to the camera body, as will as bounces of the mirror, from occurring when the mirror reaches these positions, cushions are typically used to receive the mirror at these positions so as to absorb the shocks. The shock absorbing capability of such cushions are, however, relatively limited. More effective shock absorption may be achieved by using a mirror brake mechanism which produces brake power against rotation of the mirror.
Typical mirror brake mechanisms include a friction brake comprising a disk operatively connected to and rotatable in synchronism with the mirror and a brake shoe for applying frictional brake force to the disk when the mirror is travelling in a selected angular position range. The brake shoe is urged to and in frictional contact with the peripheral edge of the disk only in a selected sectorial region thereof, which corresponds to the selected angular position range of the mirror. The selected angular position range is the range the mirror travels through when it is reaching the second position. The friction brake therefore provides brake power against the rotation of the mirror only when the mirror is reaching the second position. This facilitates smooth stopping of the mirror at the second position so as to avoid any harmful shocks and bounces.
Unfortunately, such type of mirror brake mechanisms suffer from certain inconveniences. Although the brake power to be produced by the friction brake is carefully chosen to have a desired value, the desired value itself may change depending on various factors affecting rotation of the mirror.
Such factors include, for example, temperature in the environment of the camera and orientation of the camera relative to the gravity. Specifically, when the camera is subjected to a low temperature, the viscosity of lubricant applied to pivots and sliding parts in the camera increases, resulting in an increase in resistance caused by lubricant against motions of moving parts. This in turn leads to a reduction in velocity of the mirror being rotated, and thus to a reduction in the desired brake power. In such case, lower brake power is desirable or, in other words, the actual brake power is too high to ensure appropriate motion of the mirror. Further, the brake power to be produced by the friction brake is chosen to be at an appropriate level just when the camera is in its normal orientation, where the mirror will snap up against the gravity during rotation from the first to the second position. Thus, if the camera is held by the photographer, for example, with its right or left side oriented up or down, the mirror will rotate about a vertical axis and thus not against the gravity. In such case, the velocity of the mirror when reaching the second position is relatively high so that higher brake power is desirable, and therefore the actual brake power is too low to achieve appropriate motion of the mirror.